Implantable auditory prostheses have been developed, such as cochlear implant systems, to improve impaired hearing in patients. In implantable systems, it is important to minimize the physical trauma caused by the insertion and placement of the stimulation electrode in order to reduce the risk of further hearing loss due to the insertion process itself. The use of a drug, during and/or after implantation, may help to minimize the trauma, and the stimulation electrode may be used to deliver the drug locally. For example, the stimulation electrode may be formed from a silicone material embedded with the drug or the silicone material may have a coating embedded with the drug so that the drug is released over time to the surrounding tissue. In some cases, however, it is difficult or not possible to form a thin, uniform coating with the drug that allows the release of the drug from the coating in a desired way.
U.S. Pat. No. 7,022,372 describes a method for coating an implantable device by dissolving a poly(ethylene-co-vinyl) alcohol and a drug in a solvent comprising a first solvent and a co-solvent with a low dielectric constant to form a solution. The co-solvent includes unsubstituted or substituted aliphatic hydrocarbons including n-hexane, n-pentane or cyclohexane. The patent discloses use of dimethylacetamide (DMAC) or dimethylsulfoxide (DMSO) to dissolve the polymer. The properties of the solvents (high surface energy, viscosity and degree of wetting of the substrate) can lead to less than optimal coating, and the patent discloses that the quality of the coating can be improved by adding a co-solvent. The improvement of the coating quality, when a co-solvent is added, is shown as having a low viscosity, low surface energy or low dielectric constant for the poly(ethylene-co-vinyl) alcohol. The patent discloses that the coating quality can also be improved, if a co-solvent is added having a low surface energy.
U.S. Pat. No. 5,562,922 describes that a drug can be incorporated into polyurethane by a mixture of solvent which swells (expands) but does not dissolve the polymer. An added drug can be incorporated in the interstices of the polymer. However, a swelling process would probably need long process times and a cleaning of the contact plates and areas where the drug should not be present.
U.S. Patent Appl. No. 2011/0229627 describes a method for electrospray coating of objects. The method creates electrically charged particles which then form a coating when applied to a specific substrate. The coating is prepared by sending liquid droplets containing a biologically active ingredient, a polymer dispersed in a solvent or mixture therefrom.
U.S. Pat. No. 8,030,326 discloses crystalline forms of rapamycin analogs as well as compositions, uses, and methods for aking the sa rye. A crystalline form of rapamycin is prepared, including a solvate, for instance THF, incorporated into the crystal structure. The patent describes various crystallization processes.
U.S. Patent Appl. No. 2009/0197850 describes a coating containing a drug in a polymer. The release can be controlled by the degree of crystallinity of the polymer.
WO2002/058753 discloses a method of forming a coating for an implantable device that includes forming a primer and a reservoir region. The primer region is usually used for improving the adhesion of the drug containing polymer to the implant.
U.S. Pat. No. 8,383,142 describes the use of different solvent mixtures to improve the wettability of the coating solution to the substrate usually a metal of a stent.
FIG. 1 schematically shows some components of a typical cochlear implant system in a human ear. The cochlear implant system includes an external microphone which provides an audio signal input to an external signal processor 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processor 111 into a digital data format, such as a sequence of data frames, for transmission by an external coil 107 into a receiving stimulator processor 108. Besides extracting the audio information, the receiving stimulator processor 108 may perform additional signal processing, such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through to an implanted stimulation electrode. The stimulation electrode is typically made of a flexible silicone electrode carrier with wires embedded within the silicone that are connected to stimulation contacts on the surface of the carrier. The stimulation electrode includes an electrode lead 109 and an electrode array 110, which is gently inserted into the scala tympani of the cochlea 104. Typically, the electrode array 110 includes multiple stimulation contacts 112 distributed along its surface that provide selective electrical stimulation of the cochlea 104. The electrode contacts may also be used for sensing neural tissue response signals, e.g., the stimulation electrode may also function as a measurement electrode.
Other parts of the ear are also shown in FIG. 1. The ear usually transmits sounds, such as speech sounds, through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 forms an upright spiraling cone with a center called the modiolus where the axons of the auditory nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to convert mechanical motion and energy and, in response, to generate electric pulses which are transmitted to the auditory nerve 113, and ultimately to the brain. As mentioned above, in patients with a cochlear implant system, the implanted electrode array 110 provides the electrical stimulation to the cochlea 104.